It is known that the lines of equal speed of water relative to a ship, and mainly the isotach field lines of a ship's propeller, are disturbed by two major influences: the generation of bilge vortices on the sides of the ship, and the generation of a dead water area in the wake of the ship. The vortices are generated mainly near the area where the parallel sides of the midship portion begin to curve into the stern of the ship, while the dead water zone is a region defined by the stem of the ship's hull and by the entrance angle of the water lines. Both disturbances, the vortex zone and the dead water zone, lead to losses in efficiency of propulsion, to increased wear and tear of the propeller, and to vibrations of the propeller and ship. The invention serves to minimize these disturbances and resulting disadvantages.
It has been known to conduct water in a generally sternward direction into, through and from a ship, particularly for cooling the condenser of a propeller power plant. The water has been introduced through the hull or bottom of the ship, and it has also been usual to remove such water through the bottom or hull of the ship. The cooling water is usually circulated by pumps, but has also been scooped onto and through the ship without the aid of pumps, in arrangements known to persons skilled in the art. In one such arrangement, cooling water is introduced into the ship at the approximate elevation of one-half the draft of the ship, in the region of maximum pressure in the flow of water relative to the going ship, the cooling water being removed in the approximate region of the bilge, an area of reduced pressure. Such earlier arrangement is effectively useable only in the presence of relatively thin boundary layers of the flow of marine water along the ship's hull, if the arrangement is to avoid the use of aprons and the disadvantages connected therewith. When the boundary layer is relatively thick, the pressure differential becomes insufficient for circulation of the cooling water, particularly in types of ships, which have a high planimetric ratio of propulsion, since in the operation of those types, the heavy boundary layer and the zone of flow separation from the ship's hull move approximately uniformly with the ship, without speed differential. Large ships with high planimetric propulsion ratios have boundary layers of several meters thickness, so that cooling water is hard to circulate, except by the undesirable expedient of using aprons or the like.